Manufacture of castings



Patented July 10, 1945- Philip T. Stroup and George F. Sager, New Ken- WV H slngton, Pa., assignors to Aluminum Company of America, Pittsburgh;

Pennsylvania Pa., a corporation of No Drawing. Application July 10,1942, Serial No. 450,408

5 Claims. (01. 22-200) This invention relates to methods of making sandcastings and permanent mold gravity poured castings from magnesium andthose alloys of magnesium which contains 50 per cent or more by weightof magnesium and are usually, as they are here, referred to as magnesiumbase alloys. The general object of the invention is to provide methodsof the type indicated which will, at low cost and-without material harmto the physical properties of the final product, overcome, at, least inpart, well-known difllculties encountered by the founder in the handlingof magnesium and magnesium basealloys and caused by the propensity ofsuch metal to oxidize severely when in the molten state and in contactwith air, moisture and other oxidizing media.

The invention is directed to the improvement of two well-known methodsof casting which are defined as follows:

Sand casting.-The art of forming'metal articles by pouring molten metalinto a cavity whose defining walls are formed in substantial part offoundry sand or'simllarly agglomerated materials, allowing the metal tosolidify and removing the solidified metal from the mold, such removalinvolving destruction of a large part of the walls of said cavity.

Permanent mold gravity pour casting-The art of forming metal articles bypouring molten metal into a cavity whose defining walls are formed insubstantial part of a permanent substance, such as metal. possessingcontinuity of structure or,of a semi-permanent substance, such-asplaster. or a pressure consolidated agglomerate, allowing the metal tovsolidify and removing the solidified metal from the mold, such removalinvolving no destruction of any large part of the cavity-formingsurfaces. The product may be a shaped casting or an ingot for working,such as a rolling or extrusion ingot. The mold cavity may be of anyshape including those molds without top or bottom, such as are used incontinuous casting processes. It is emphasized here that the moltenmetal proceeds into the cavity only under the force imparted to it bygravity, thus distinguishing from the use of pressure on the flowingmolten quality castings therefrom under conditions which would justifythe commercial production of sand casting and permanent mold gravitypoured castings. Gradual development of methods of preventing oxidationduring casting finally led to the commercial production of castings ofthese types, but the methods necessarily used in their pro-' 'ductionadd materially to the cost of manufacture and often have detrimentaleffects upon the properties of the final product. If the molten metal istoibe molded by sand casting, the sand molds are either completely dried(an expensive and unsatisfactory expedient) or there are mixed with thesand, or similar agglomerate,

special inhibitors and tempering agents designed to protect againstoxidation of the metal during solidification in the mold cavity. Suchinhibitors and agents are well known to the art. Theylnclude suchsubstances andcompounds as sulphur and compounds thereof, fluorides,fiuosilicates and other compounds of fluorine, boric acid, ammonia,glycerine, glycols and many other materials, most of which produceirritating fumes during the casting process and are difficult andexpensive to handle. In addition to such costly procedures, calcium issometimes added to the molten metal since the presence of calcium tendsto decrease the propensity of the metal to oxidize. Under'theseconditions it is possible to produce satisfactory magnesium andmagnesium base alloy sand castings in specially equipped foundriesdespite the fact that the foundry sand must be continuallyreconditioned, the operators must contend with the acrid fumes given offby the in:- hibitors and the action of the inhibitors causes theformation of voids and gas pockets in the surfaces of the castings. g a

In the production of permanent mold gravity poured castings the methodsused to protect the molten magnesium and magnesium base alloy againstoxidation are somewhatdifl'erent. The continuity of structure whichcharacterizes the Walls of the mold cavity in this process preventsincorporation of inhibitors in the cavity surface. Inhibitors, if used,must be ingaseous form, such as, for instance, sulphur vapor or sulphurdioxide. The use of such gases is cumbersome and interferes with thecomparative rapidityof production which is a. feature of this type ofcasting process. Consequently, some founders avoid the use of suchinhibitors and instead provide the molten metal with a calcium contentand also treat the casting cavity walls with mold washes designed tocoat these walls and to thusprevent action between cavity wall and'molten metal during the casting operation. This involves difficulties.Calcium will itself oxidize or burn while in the molten metal, thusnecessitating frequent additions of that element to the molten magnesiumor magnesium base alloy which is be- 5 ing processed. As a result, theexact amount 0L- calcium present in the melt at any given time is notknown to the operator until a deficiency occurs, in which case themolten metal suffers severe oxidation and interferes with the castingprocess. The mold wash coating On the cavity walls is not stable andattack thereon by the molten metal necessitates constant renewing ofthis coating, thus detracting from the emciency of the casting process.Evenunder the best of these conditions the castings produced often haveblack or dark discolored surfaces and show skin folds and similarsurface defects. 7 7

The present invention, which provides methods designed to overcome ordiminish the difilculties above mentioned, is predicated upon thepresence in the magnesium or magnesium base alloy during sand casting orpermanent mold gravity pour casting of certain small quantities ofberyllium. This results in advantages hereinafter described. Prior tothis invention it was known that beryllium acted as an anti-oxidant inmagnesium and magnesium base alloy, but attempts to realize upon theanti-oxidant properties in sand casting or permanent mold gravity pourcasting had failed because the resultant castings had poor physicalcharacteristics; We have discovered that the failure of prior workers inthis field was ,caused by the use of improper amounts of beryllium(0.005 or more percent by weight). We have further discovered that whenberyllium is used in the proper amounts hereinafter set forth, it ispossible not only to utilize its anti-oxidation effect in sand castingand in permanent mold gravity pour casting, 4 but to also obtain resultsand advantages heretofore unknown."

When our, invention is applied to the art of sand casting the magnesiumor magnesium base alloy to be cast.is provided with a beryllium 5content of not more than about 0.002 or less than 0.00005 per cent byweight of the total metal and cast into the mold cavity. The tendency ofthe molten metal to react with moisture or other oxidizing media presentin the molding sand is so substantially reduced that the amount ofinhibitor heretofore necessarily present in the sand may besubstantially decreased and in some cases, particularly in the case ofcore sands, eliminated. The necessity for the addition of calcium to themetal is also eliminated. The cost of processing is therefore reduced,the number of castings rejected for surface voids is also reduced. Theelimination of a part of the inhibitor previously used also reduces theconcentration of acrid 6o fumes therefrom aand betters the workingconditions in the foundry. We have consistently obtained these resultson a commercial scale without substantial decrease in the properties ofthe resultant castings, particularly when, as is es preferred, theamount of beryllium present in the metal does not exceed about 0.0015per cent by weight.

When our invention is applied to the art of permanent mold gravity pourcasting the magnesium or magnesium base alloy to be cast is providedwith a beryllium content of not more than about 0.002 or less than about0.00005 per cent by weight of the total metal and cast into the moldcavity. The calcium previouslyv used be positively added thereto.

in such processes can be dispensed with completely or in large part andthus the uncertainties due to the use .of calcium can be eliminatedsince the beryllium content is stable throughout the casting operation.The presence of beryllium also protects the mold wash coating on thecavity surfaces, and when our invention is practiced it is not necessaryto so frequently interrupt the process to renew this coating. Thecastings produced do not exhibit the dark unsightly surface of castingspreviously produced and the surface thereof contains less skin folds andother surface defects than the castings of prior processes of this type.The final casting has properties not materially affected, if at all, bythe practice of the invention. In fact, the elimination of the necessityfor any substantial calcium content makes possible the production ofcastings which will respond uniformly to thermal treatment and whichhave, as the result of the thermal treatment, better properties thanwere obtained in previous castings of equal composition, which containedcalcium and no beryllium or beryllium in amount heretofore proposed inthe art.

The minute amounts of beryllium used inv the practice of the inventionas above described are,

despite the fact that they are of an order usually disregarded as atrace of an element, not present in primary magnesium metal and mustWhere secondary magnesium metal or alloy is available and containsberyllium, it may, of course, be used and if the beryllium content isnot within the range postulated by this invention, adjustment thereofmay first be made in any suitable manner.

Where beryllium is to be added to magnesium or magnesium base alloy themethod of. addition is not important, the factor of importance beingthat the proper quantity is achieved in the metal. An intermediate alloyof the beryllium and magnesium or of beryllium and one or more of theother elements of which the alloy is to be composed can be used as asource of beryllium. Another method consists in adding. to the moltenmagnesium or magnesium base alloy a beryllium salt decomposable at thetemperature of the molten metal. Other methods will suggest themselvesto those skilled in the art of alloying.

The magnesium base alloys referred to herein and in the appended claimsare those alloys containing at least 50 per cent and usually more byweight of magnesium and having as additional constituents, apart fromthe beryllium, other elements such as manganese, zinc, cadmium,aluminum, silicon, copper, cerium, zirconium, silver and such others asare usable or used to produce useful properties in, or to enhance theproperties of, magesium.

We claim: 1. In th sand casting of, magnesium and magnesium base alloy,the step of providing the metal prior to the casting operation with acontent of beryllium not greater than about 0.002

and not less than 0.00005 per cent by weight of the total metal.

2. In the sand casting of magnesium and magnesium base alloy, the stepof providing the metal prior to the casting operation with a content ofberyllium not greater than about 0.0015 and not less than0.00005 percent by weight of the total metal.

3. In that process of sand casting magnesium and magnesium base alloywhich comprises mixgreater than about 0.002 and not less than about0.00005 per cent by weight of the total metal.

5. In the gravity pour casting of magnesium and magnesium base alloy,the step of providing the metal prior to the casting operation with acontent of beryllium not greater than about 0.002 per cent and not lessthan about 0.00005 per cent by weight of the total metal.

PHILIP T. STROUP. GEORGE F. SAGER.

